Serum potassium is an electrolyte whose concentration - in order to guarantee a negative balance - varies rapidly and significantly during dialysis, frequently resulting in going from pre-dialysis hyperpotassaemia to intra-dialysis hypopotassaemia. As mentioned above, in Dolson's study [1], differences in dialyses blood pressure were not found between the groups treated with dialysates containing 1, 2 or 3 mmol/l of potassium, but at the end of the dialyses those patients treated with the lower potassium concentrations showed what was called a "rebound hypertension".